1. Field of the Invention
The present invention relates to a liquid crystal display device which uses an AFLC (AntiFerroelectric Liquid Crystal), and, more particularly, to an AFLC display device which can ensure gradation display.
2. Description of the Related Art
A ferroelectric liquid crystal (FLC) display device which uses a ferroelectric liquid crystal has a faster response and wider angle of visibility as compared with a TN liquid crystal display (LCD) device which uses a nematic liquid crystal.
Known FLC display devices include an FLC display device which uses an FLC and an AFLC display device which uses an AFLC.
The AFLC display device displays images using the stability of the alignment states an AFLC has.
This will be discussed more specifically. An AFLC has three stable alignment states of liquid crystal (LC) molecules. (1) When a voltage equal to or greater than a first threshold value is applied to the AFLC, the liquid crystal is set to a first ferroelectric phase where the LC molecules are aligned in a first direction or to a second ferroelectric phase where the LC molecules are aligned in a second direction according to the polarity of the applied voltage. (2) When a voltage equal to or smaller than a second threshold value lower than the first threshold value is applied to the AFLC, the liquid crystal is set to an antiferroelectric phase as an intermediate aligning state between the first and second ferroelectric phases. By setting the directions of the transmission axes of a pair of polarization plates located on both sides of the LCD device based on the optical axis in the antiferroelectric phase as a reference, it is possible to display images by controlling the transmittance of light in accordance with the applied voltage, as shown in FIG. 39.
The AFLC stays set to the first or second ferroelectric phase or the antiferroelectric phase even if the applied voltage changes as long as the variation lies between the first and second threshold values. That is, the AFLC has a memory property. The conventional AFLC display device is driven in a simple matrix mode using this memory property.
The memory property of the AFLC is determined by the difference between the voltage which causes the liquid crystal to change its phase to the antiferroelectric phase from the first or second ferroelectric phase and the voltage which causes the liquid crystal to change its phase to the first or second ferroelectric phase from the antiferroelectric phase. The greater this voltage difference becomes, the higher the memory property of the alignment state gets. That is, the memory property becomes greater as the hysteresis of the optical characteristic increases.
The conventional AFLC display device which is driven in a simple matrix mode therefore uses an AFLC whose voltage difference is large. But, it is not possible to arbitrarily control the light transmittance of the conventional AFLC display device which uses an AFLC having a high memory property. That is, it is extremely difficult to control the display gradation, so that multi-gradation display cannot be accomplished.